1. Field of the Invention
The present invention relates to techniques for performing wellbore operations. More particularly, the present invention relates to techniques for determining downhole characteristics, such as electrical parameters of downhole fluids and/or subterranean formations.
2. Background of the Related Art
Oil rigs are positioned at wellsites for performing a variety of oilfield operations, such as drilling a wellbore, performing downhole testing and producing located hydrocarbons. Downhole drilling tools are advanced into the earth from a surface rig to form a wellbore. Drilling muds are often pumped into the wellbore as the drilling tool advances into the earth. The drilling muds may be used, for example, to remove cuttings, to cool a drill bit at the end of the drilling tool and/or to provide a protective lining along a wall of the wellbore (or borehole). During or after drilling, casing is typically cemented into place to line at least a portion of the wellbore. Once the wellbore is formed, production tools may be positioned about the wellbore to draw fluids to the surface.
During drilling, measurements are often taken to determine downhole conditions. In some cases, the drilling tool may be removed so that a wireline testing tool may be lowered into the wellbore to take additional measurements and/or to sample downhole fluids. Once the drilling operation is complete, production equipment may be lowered into the wellbore to assist in drawing the hydrocarbons from a subsurface reservoir to the surface.
The downhole measurements taken by the drilling, testing, production and/or other wellsite tools may be used to determine downhole conditions and/or to assist in locating subsurface reservoirs containing valuable hydrocarbons. Such wellsite tools may be used to measure downhole parameters, such as temperature, pressure, viscosity, resistivity, etc. Such measurements may be useful in directing the oilfield operations and/or for analyzing downhole conditions.
Various techniques have been developed for measuring downhole parameters as described, for example, in U.S. Pat. Nos. 6,801,039, 6,191,588, 6,919,724, 7,066,282, 6,891,377, 5,677,631, 5,574,371, 4,567,759, and 3,816,811. In some cases, techniques have been generated for determining parameters of the formations surrounding the borehole. For example, micro-resistivity measurements of borehole walls may be taken to generate images of formations surrounding the borehole. Such micro-resistivity measurements may be taken using downhole tools, such as a Fullbore Micro Imager (FMI™) of SCHLUMBERGER™ and an Earth Imager™ of BAKER ATLAS™. In one example, measurements may be taken using current injection when the borehole is filled with a conductive fluid or mud. Where a non-conductive fluid is present, such as oil-based mud (OBM) with a very high resistivity compared to that of the formation such that a thin layer of mud between a measurement electrode and the formation, high impedance is generated between the electrode and the formation. Another example mounts one or more button voltage electrodes on an insulating pad, such as is used in the Oil Base Micro Imager tool (OBMI™) of SCHLUMBERGER™.
Stability problems may sometimes occur in cases where a measurement electrode touches the formation, or if the mud has conductive bubbles in it which form a low-impedance electrical connection between the measurement electrode and the formation. High impedance between the electrode and the formation can suddenly reduce to very small impedance or vice versa, which may lead to a change in the measurement that is not due to a change in formation parameters. For example a small change from 0.10 mm to 0.00 mm mud thickness can lead to a significant change in impedance. In general, both the magnitude and the phase of the impedance can change drastically.
It may be desirable in some cases to provide a minimum distance or stand-off between a measurement pad and the borehole wall. Attempts have been made to provide protruding elements, for example protruding wear plates, on the sensor pad to touch the formation and keep the pad's front face away from the formation. However, existing protruding devices may be subject to damage in downhole conditions, may still have problems with measurements where conductive bubbles are present in the mud, and may be subject to large standoff variations during the logging process.
The invention contained herein is provided to address deficiencies with existing techniques.